Worm Breeder's Gazette 7(2): 49
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In the last Newsletter we described the phenotypes and map positions
of some new dominant mutations. To explore the nature of genes that
can mutate to dominant alleles, we have begun to attempt to determine
the null phenotypes of a set of such genes. Since dominance often
reflects the synthesis of a gene product altered in function or
increased in level (e.g., see Greenwald and Horvitz, Genetics, 96, 147,
1980; also, 110, in press), the introduction of loss of function (
'null') mutations should generate alleles that are recessive. Thus,
the intragenic reversion of dominant alleles offers one approach to
the isolation of null mutations.
We have attempted to obtain intragenic revertants of mutants in
seven genes with dominant alleles. In general, F1 and F2 progeny of
EMS-mutagenized homozygous hermaphrodites were screened for revertants
('homozygous reversion'). If this protocol failed to produce tightly-
linked (presumably intragenic) revertants, as it must for certain
genes (e.g., those with null phenotypes that are either lethal or
indistinguishable from the dominant phenotypes), N2 males were mated
with EMS-mutagenized homozygous hermaphrodites and wild-type F1
progeny sought ('heterozygous reversion'). Only tightly-linked
revertants (less than 0.1% from the original mutation), which we have
tentatively assumed to be intragenic, are described below.
The following table summarizes the number of revertants and the
homozygous phenotypes of the putative intragenic revertants obtained
from these protocols.
[See Figure 1]
Mutants in unc-105, to
homozygous viable phenotypically wild-type animals. Mutants in unc(
n498) and unc(n494) 'reverted' from one Unc phenotype to another: n498
animals are almost paralyzed, where as n498 revertants are only mildly
uncoordinated; n494 animals are very sluggish and assume coiled
postures, whereas n494 revertants are kinky and relatively active.
unc(n493) reverted only as a heterozygote, and the revertant
heterozygotes segregated larvae that failed to develop. All of these
revertants are recessive, and all appeared at the frequency expected
after 4 EMS-mutagenesis for null mutations in a typical C. elegans
gene (10+E-3 to 10+E-4); in addition, spontaneous intragenic
revertants, which are likely to reveal null phenotypes (Greenwald and
Horvitz, 1980), have been obtained for unc-105,
n498). Thus, it is likely that these
revertants display the null phenotypes of these genes (although for
unc(n498) and unc(n494) it remains possible that the null phenotype is
wild-type, and the original strains contain tightly-linked recessive
unc mutations). That rol-6 and unc-8 have wild-type null phenotypes
is further supported by the observation that wild-type intragenic
revertants failed to complement visible recessive alleles (e187 and
e49, respectively) of these loci. We have been unable to isolate
revertants of n501 by either protocol, suggesting that n501 either
itself reduces gene activity or defines a locus that is relatively
difficult to mutate with EMS.